With fresh peppermint:
Put 1 cup distilled or other quality water in non-reactive pan and add 1/2 cup or so packed peppermint leaves, bruising the leaves with a spoon as you place them in the water. Bring to a simmer; cover and let leaves steep until water is cool. Strain and bottle in sterilized spray bottle (or simply put the bottle and sprayer on the top rack of the dishwasher and the high heat of the dishwasher should clean the bottle well. Store in refrigerator. With fresh lavender: Same method as above, using both leaves and flowers if you have them. Pick lavender flowers before noon since the essential oils are driven out with the heat.
With essential oils: Anywhere from 8 to 12 oz distilled water in sterilized spray bottle. (See above for sterilizing). Add peppermint or lavender esssential oil in drops; starting with about 10 drops and going from there, depending upon the size of the drops. You'll want a very fragrant aroma. Store in refrigerator.